Most electric water heaters use an electric resistant rod in the tank. But there is another option for electric water heaters – one that is familiar to most Floridians, and that is a dedicated heat pump for heating hot water. Just like a regular heat pump for heating the air in your house, the heat pump water heater has a small compressor unit on top that uses vapor compression to heat the water. Prior FSEC research on heat pump water heaters (HPWH) in Florida showed that they saved approximately 66% of the energy needed to heat water with an electric resistance system. HPWHs also create a quantity of cooled, dehumidified air from the compressor section of the unit as a by-product of their operation. FSEC researchers found out that a HPWH coupled to the conditioned living space can reduce space-conditioning energy in a cooling-dominated climate, but with qualifications a lab test was undertaken to investigate the effect of coupling a garage located HPWH to the conditioned space with ductwork. With the HPWH ducted to and from the interior, cooling energy dropped by 4% or 0.8 kWh/day. Effect on space heating energy for this configuration could not be determined. Experiments also investigated using an outdoor air source for the HPWH, to supplement ventilation. During the cooling season, the HPWH tempered the outdoor air with only a minimal impact on cooling energy. Space heating energy increased by 18% or 1.4 kWh/d. The space coupling of the HPWH had a minimal impact on water heating efficiency.
In later field evaluation, eight occupied homes were retrofitted with a HPWH coupled to the conditioned space. Results were more pronounced than the lab evaluation: cooling energy savings averaged 8% (1.1 kWh/day). Space heating energy use increased by 24%, although with considerable variation and little application in Florida’s mild climate. The evaluation suggested the coupling eroded some of the HPWH water heating energy use savings, reducing it by 0.4 kWh/day or 11%. If not located in the house, you often end up with a slightly cooler garage.
*This paper was presented at the 2016 ACEEE Summer Study on Energy Efficiency in Buildings in Pacific Grove, CA on August 22 -26, 2016.